There are conventionally known blowers in which a cross-flow fan or other type of multi-blade fan is used, wherein wind noise is produced by multiple blades. To counteract a wind noise component having a fundamental frequency related to the number of rotations N and the number of blades Z (referred to below as “NZ noise”) from within the wind noise, values of the angle of the pitch between the blades of the cross-flow fan are arranged at random (random pitch angle arrangement), whereby the inter-blade pitch angle arrangement is varied to reduce noise. Such variation of the inter-blade pitch angle arrangement produces increases/decreases and/or time distortion in acoustic-pressure fluctuation, which causes the NZ noise, to offset the timing at which the NZ noise is generated, making it possible to minimize increases in unpleasant noise by reducing the prominence of NZ noise having a characteristic frequency.
However, in conventional methods for determining such inter-blade pitch angle arrangements randomly, the amount by which the NZ noise is reduced changes for each determination of the arrangement, resulting in an unpredictable, ad-hoc method of solution.
Furthermore, there are many cases in which the randomly determined arrangement coincidentally matches an inter-blade pitch angle arrangement in which noise is prominent at low frequencies; in order to obtain an optimal arrangement in which noise prominent at low frequencies is suppressed while significantly reducing NZ noise, it is necessary repeatedly to perform a process of trial-and-error. This is not an efficient method for determining the inter-blade pitch angle arrangement for blowers in which the cross-flow fans have different specifications, such as with respect to number of blades.
In the method for determining inter-blade pitch angle arrangement described in, e.g., Japanese Patent No. 3484854, an arrangement is imparted such that a sine waveform of a particular order is obtained when the inter-blade pitch angle arrangement is expanded in a Fourier series. When the inter-blade pitch angle arrangement is determined in this manner, the NZ noise is linked to the reduction of low-frequency broadband noise.